1. Field of the Invention
The present invention relates to a spin-valve type magnetoresistive sensor wherein electrical resistance is changed depending on the relationship between the magnetization direction of a pinned magnetic layer and the magnetization direction of a free magnetic layer affected by an external magnetic field. More particularly, the present invention relates to a spin-valve type magnetoresistive sensor which has higher sensitivity of detection and is adaptable for high-density recording as the result of an improvement in structure and material properties of a spin-valve film laminate, as well as to a spin-valve type magnetoresistive head using the sensor.
2. Description of the Related Art
There are known spin-valve type and multilayer type as laminated structures capable of developing a GMR (Giant Magnetoresistive) effect.
FIG. 12 is a sectional view showing a conventional multilayer type GMR sensor.
The multilayer type GMR sensor has a laminated structure comprising pairs of a ferromagnetic material layer 9 and a non-magnetic electrically conductive layer 2 which are formed in plural number repeatedly from the bottom.
Generally, the ferromagnetic material layer 9 is made of a NiFe (nickel-iron) alloy or a CoFe (cobalt-iron) alloy, and the non-magnetic electrically conductive layer 2 is made of Cu (copper).
The ferromagnetic material layers 9 are positioned over and under the non-magnetic electrically conductive layer 2 in a laminated structure. Particularly, when the non-magnetic electrically conductive layer 2 is formed in a thickness on the order of 10-20 angstroms, the upper and lower ferromagnetic material layers 9 are magnetized into a single domain state in anti-parallel relation uniformly due to the RKKY interaction.
In the multilayer type GMR sensor, when the sensor is subject to a leakage magnetic field from a magnetic recording medium such as a hard disk, the magnetization direction of the ferromagnetic material layer 9 is varied to the same direction as the leakage magnetic field. A variation in the magnetization direction of the ferromagnetic material layer 9 changes electrical resistance, and this change in value of the electrical resistance results in a voltage change. The leakage magnetic field from the magnetic recording medium is detected based on the resulting voltage change.
Meanwhile, a magnetoresistance ratio (MR ratio) of the multilayer type GMR sensor amounts to the order of about 10-30% when an external magnetic field is in the range of several tens Oe (oersted) to several thousands Oe. The reason why the magnetoresistance ratio has a very large value is that there are a very large number of places where electrons scattering can occur. Further, a very strong external magnetic field is required to achieve such a high magnetoresistance ratio. This is because the magnetization direction of the ferromagnetic material layer 9 is firmly fixed in anti-parallel relation due to the RKKY interaction. It has been found from calculation of plane recording density based on the magnetoresistance ratio in the above range that the multilayer type GMR sensor is adaptable for the plane recording density up to value on the order of 100 Cb/in.sup.2. But it has also been confirmed that when a relatively weak external magnetic field on the order of several Oe is applied, the magnetoresistance ratio of the multilayer type GMR sensor becomes smaller than that of a spin-valve type magnetoresistive sensor.
FIG. 13 shows a conventional single spin-valve type magnetoresistive sensor. This sensor comprises four layers, i.e., a free magnetic layer 1, a non-magnetic electrically conductive layer 2, a pinned magnetic layer 3 and an antiferromagnetic layer 4 from the top. Numerals 5, 5 on both sides denote hard bias layers. Denoted by 6, 7 are respectively a buffer layer and a barrier layer made of non-magnetic material, such as Ta (tantalum), and 8 is an electrically conductive layer. The pinned magnetic layer 3 is selected to have a greater coercive force than the free magnetic layer 1.
Because the pinned magnetic layer 3 and the antiferromagnetic layer 4 are formed in contact with each other, the pinned magnetic layer 3 is put into a single domain state in the Y-direction and has the magnetization direction fixed in the Y-direction under an exchange anisotropic magnetic field produced by exchange coupling at the boundary surface between the pinned magnetic layer 3 and the antiferromagnetic layer 4. By heat-treating (annealing) the sensor under a magnetic field applied thereto, the exchange anisotropic magnetic field can be produced at the boundary surface between the pinned magnetic layer 3 and the antiferromagnetic layer 4.
Also, the hard bias layers 5 magnetized in the X-direction affects the free magnetic layer 1 so that the magnetization direction of the free magnetic layer 1 is uniformly set in the X-direction. In other words, since the free magnetic layer 1 is put into a single domain state in the predetermined direction by the presence of the hard bias layers 5, the occurrence of Barkhausen noise can be prevented.
In the above single spin-valve type magnetoresistive sensor, a steady electric current is applied from the electrically conductive layers 8 to the free magnetic layer 1, the non-magnetic electrically conductive layer 2 and the pinned magnetic layer 3. A magnetic recording medium such as a hard disk runs in the Z-direction. When a leakage magnetic field from the magnetic recording medium is applied to the sensor in the Y-direction, the magnetization direction of the free magnetic layer 1 is varied from the X-direction to the Y-direction. Thus, electrical resistance is changed depending on the relationship between a variation in the magnetization direction of the free magnetic layer 1 and the fixed magnetization direction of the pinned magnetic layer 3. This change in value of electrical resistance result in a voltage change. The leakage magnetic field from the magnetic recording medium is detected based on the resulting voltage change.
FIG. 14 is a sectional view showing a conventional dual spin-valve type magnetoresistive sensor.
In the dual spin-valve type magnetoresistive sensor, non-magnetic electrically conductive layers 2, 2, pinned magnetic layers 3, 3 and antiferromagnetic layers 4, 4 are formed into laminated structures on both sides of a free magnetic layer 1 at the middle in vertically symmetric relation. The magnetization direction of the free magnetic layer 1 is uniformly set in the X-direction by the presence of hard bias layers 5 magnetized in the X-direction. Also, the pinned magnetic layers 3, 3 are each put into a single domain state in the Y-direction and have the magnetization direction fixed in the Y-direction under an exchange anisotropic magnetic field produced by exchange coupling at the boundary surface between itself and the antiferromagnetic layer 4.
When a leakage magnetic field from the magnetic recording medium is applied to the sensor in the Y-direction, the magnetization direction of the free magnetic layer 1 is varied from the X-direction to the Y-direction, whereupon a value of electrical resistance is changed.
In the spin-valve type magnetoresistive sensor, when the magnetization direction of the free magnetic layer 1 is varied from the X-direction to the Y-direction, electrons existing between the free magnetic layer 1 and the pinned magnetic layer 3 and tending to move from one to the other are scattered at the boundary surface between the non-magnetic electrically conductive layer 2 and the free magnetic layer 1 and at the boundary surface between the non-magnetic electrically conductive layer 2 and the pinned magnetic layer 3. As a result, the value of electrical resistance is changed and the leakage magnetic field from the magnetic recording medium is detected based on the resulting voltage change.
The electrical resistance shows a maximum value when an angle formed between the magnetization direction of the free magnetic layer 1 and the magnetization direction of the pinned magnetic layer 3 is maximized, i.e., when these two layers are magnetized in anti-parallel relation, and shows a minimum value when the magnetization direction of the free magnetic layer 1 and the magnetization direction of the pinned magnetic layer 3 are the same. Thus, as a magnetoresistance ratio, i.e., {(maximum voltage value-minimum voltage value)/minimum voltage value}, has a larger value when subject to the leakage magnetic field from the magnetic recording medium, the spin-valve type magnetoresistive sensor and hence a spin-valve type magnetoresistive head using the sensor have better characteristics.
Further, a detection output of the leakage magnetic field also greatly depends on the magnitude of a steady electric current (sensing electric current). The larger the steady electric current, the larger will be the detection output. However, if the density of the electric current flowing through the free magnetic layer 1, the non-magnetic electrically conductive layer 2 and the pinned magnetic layer 3 is too large, resulting Joule heat would give rise to problems of reducing the detection output, reliability and durability, and hence deteriorating characteristics of the spin-valve type magnetoresistive sensor. It has been confirmed that an upper limit value of the steady electric current allowing the spin-valve type magnetoresistive sensor to have satisfactory characteristics is 3.times.10.sup.7 A/cm.sup.2. Incidentally, the upper limit value of the steady electric current can be raised by increasing the total number of layers making up the structure of the spin-valve type magnetoresistive sensor.
In the single spin-valve type magnetoresistive sensor shown in FIG. 13, electron scattering occurs at two places, i.e., the boundary surface between the non-magnetic electrically conductive layer 2 and the free magnetic layer 1 and the boundary surface between the non-magnetic electrically conductive layer 2 and the pinned magnetic layer 3. On the other hand, in the dual spin-valve type magnetoresistive sensor shown in FIG. 14, electron scattering occurs at four places in total, i.e., the two boundary surfaces between the non-magnetic electrically conductive layers 2 and the free magnetic layer 1 and the two boundary surfaces between the non-magnetic electrically conductive layers 2 and the pinned magnetic layers 3. Therefore, the dual spin-valve type magnetoresistive sensor has a larger magnetoresistance ratio than the single spin-valve type magnetoresistive sensor.
To make the sensor adaptable for high-density recording, it is important to improve the plane recording density. To improve the plane recording density, it is required to increase an output reproduced by the sensor. Further, to increase the reproduction output, it is required to raise the magnetoresistance ratio which is proportional to the reproduction output.
As mentioned above, the multilayer type GMR sensor shown in FIG. 12 can provide the magnetoresistance ratio of about 30% at maximum when the external magnetic field of several thousands Oe is applied to the sensor. However, when the external magnetic field is very weak, the magnetoresistance ratio of the multilayer type GMR sensor becomes smaller than that of the spin-valve type magnetoresistive sensor.
Further, for the multilayer type GMR sensor, it is impossible to provide the hard bias layers and suppress the occurrence of Barkhausen noise unlike the spin-valve type magnetoresistive sensor.
The reason is that if the hard bias layers are provided in the multilayer type GMR sensor, all the layers of ferromagnetic material would be magnetized uniformly in the same direction as the magnetization direction of the hard bias layers, and hence that if the external magnetic field is applied to the multilayer type GMR sensor, the electrical resistance would not be changed and the sensor could not detect the leakage magnetic field of the magnetic recording medium.
In the conventional spin-valve type magnetoresistive sensors, generally, the free magnetic layer 1 and the pinned magnetic layer 3 are each formed of, e.g., a FeNi (iron-nickel) alloy film, and the non-magnetic electrically conductive layer 2 is formed of, e.g., a Cu (copper) film. In the conventional single spin-valve type magnetoresistive sensor, a FeMn (iron-manganese) alloy film is generally used as antiferromagnetic material of the antiferromagnetic layer 4.
However, the FeMn film has a disadvantage that it is susceptible to corrosion and would be rusted soon if exposed to air containing moisture. Further, the blocking temperature for exchange coupling between the FeMn alloy film as ferromagnetic material and the FeNi alloy film constituting the pinned magnetic layer is as low as on the order of about 150.degree. C. This raises another disadvantage that if the temperature of a magnetoresistive head becomes high due to the heat generated by itself and the ambient temperature during operation, the exchange anisotropic magnetic field is weakened and noise in the detection output is increased.
As ferromagnetic material substitutable for the FeMn alloy, there are an IrMn (iridium-manganese) alloy, a RhMn (rhodium-manganese) alloy, etc.
However, films of the FeMn (iron-manganese) alloy, the IrMn (iridium-manganese) alloy, the RhMn (rhodium-manganese) alloy, etc. have properties as follows. When these alloy films are formed over the ferromagnetic material, such as an FeNi alloy, constituting the pinned magnetic layer 3, any alloy film can develop exchange coupling at the boundary surface between itself and the pinned magnetic layer 3. But those antiferromagnetic materials have such features that they are easily affected by underlying layers and their films are hard to exhibit antiferromagnetic characteristics in the vicinity of their upper surfaces. Accordingly, when the pinned magnetic layer 3 is formed over any film of those antiferromagnetic materials, it is difficult for the film of the antiferromagnetic material to develop exchange coupling.
On the contrary, films of other antiferromagnetic materials such as a CoO (cobalt oxide) alloy and a NiO (nickel oxide) alloy can each develop exchange coupling at the boundary surface between itself and the pinned magnetic layer 3 when the film is formed under the ferromagnetic material constituting the pinned magnetic layer 3. However, the films of those antiferromagnetic materials such as a CoO alloy and a NiO alloy have a feature of showing dependency in degree of crystallinity. More specifically, each film of those antiferromagnetic materials is hard to develop a satisfactory degree of crystallization in the vicinity of the boundary surface between itself and an underlying layer at start-up of a vacuum film forming process using the sputtering method, for example, attains better growth of crystals at a position farther away from the buffer layer, and hence has a difficulty in exhibiting satisfactory antiferromagnetic characteristics in the vicinity of its lower surface. Accordingly, when the pinned magnetic layer 3 is formed under any film of those antiferromagnetic materials, it is difficult for the film of the antiferromagnetic material to develop exchange coupling.
Thus, the above-mentioned antiferromagnetic materials can develop effective exchange coupling only when formed on one side of, i.e., either over or under, the pinned magnetic layer 3. Therefore, the above-mentioned antiferromagnetic materials cannot be employed in the structure of the dual spin-valve type magnetoresistive sensor shown in FIG. 14 wherein the antiferromagnetic layers 4, 4 are formed over and under the pinned magnetic layers 3, 3.
There is known a NiMn (nickel-manganese) alloy as material capable of producing an exchange anisotropic magnetic field regardless of whether its film is formed over or under the pinned magnetic layer 3. This antiferromagnetic material can be formed on both sides of, i.e., over and under, the pinned magnetic layer 3 and can be used in the dual spin-valve type magnetoresistive sensor shown in FIG. 14.
To develop effective exchange coupling between a NiMn alloy film and a FeNi alloy film (pinned magnetic layer 3), however, annealing is required to be carried out at a relatively high temperature. Generally, to produce an exchange anisotropic magnetic field, it is necessary to apply a magnetic field and carry out annealing after the antiferromagnetic layer 4 and the pinned magnetic layer 3 are formed in contact relation. In the case where the antiferromagnetic layer 4 is formed of a NiMn alloy film and the pinned magnetic layer 3 is formed of a FeNi alloy film, the annealing temperature as high as about 250.degree. C. or above is required to develop effective exchange coupling therebetween.
But the annealing at a high temperature of about 250.degree. C. or above would raise a problem below. There occurs diffusion of metallic elements at the boundary surfaces of the free magnetic layer 1 and the pinned magnetic layer 3 adjacent to the non-magnetic electrically conductive layer 2 of Cu. This affects the magnetoresistive effect due to electron diffusion occurred at the boundary surface between the free magnetic layer 1 and the non-magnetic electrically conductive layer 2 and the boundary surface between the pinned magnetic layer 3 and the non-magnetic electrically conductive layer 2. The magnetoresistance ratio depending on the external magnetic field is thereby reduced.
Meanwhile, to make the sensor adaptable for high-density recording, it is important to not only improve the plane recording density, but also reduce a magnetic gap length G1. When the antiferromagnetic layer is formed of a NiMn alloy film, a good exchange anisotropic magnetic field cannot be achieved unless the antiferromagnetic layer must have a film thickness on the order of several hundreds angstroms. Therefore, a thickness h' of the multilayered films shown in FIG. 14 is necessarily increased and hence the magnetic gap length G1 cannot be made small. Incidentally, the free magnetic layer 1, the non-magnetic electrically conductive layer 2 and the pinned magnetic layer 3 have each a film thickness on the order of several tens angstroms.
The magnetoresistance ratio of the single spin-valve type magnetoresistive sensor is in the range of 3 to 9%. Calculation of plane recording density based on the magnetoresistance ratio in the above range results in that the single spin-valve type magnetoresistive sensor is adaptable for the plane recording density up to values on the order of 10 Cb/in.sup.2.